I was dealing with this same problem this afternoon on a '91 240 wagon. I have the passenger door lock and tailgate lock unresponsive, and a 250mA battery drain. I was figuring the drain and lock problems are connected. I took out the lock cylinder and plunger switches after inspecting the wiring as far as possible while the parts were installed in the door. I saw no sign of insulation wear while installed. I took apart the lock cylinder collar switch to clean the contacts and noticed that the fine multistrand wires are a bit fluffed up where they attach to the silver contact rings. Some of the strands from one wire can and do touch strands from the adjoining wires.

This is all where the three wires enter the plastic housing, which Will said he super glued. His must not have been as fuzzy as mine because that would not help in my case. I undid tabs on the silver strips and managed to pull them out. Then I ed errant strands of wire together and soldered them a bit. Not so much that the entire exposed part was tinned, just the end touching the silver. The wires seem to be spot welded to the silver strips at the factory.

After putting the switch back together with cleaned contacts and "more solid" wire joints, and reinstalling, I found much crisper action for the door locks and even heard the tailgate lock activate. However, the news was not all good. It is working intermittently. If I wiggle the wires where they enter the plastic housing of the lock cylinder switch the locks sometimes work, and sometimes don't. That switch is still having internal shorting problems, just not quite as bad as before.

It is about a $100 part where I've seen it listed online. Darn.